1. Field of the Invention
This invention relates to the field of optical systems, and more particularly to the method and apparatus for receiving optical information.
2. Description of Related Art
Infrared detecting systems often use optical systems to focus electromagnetic radiation onto an infrared detector array. For those applications where the spectral range of the radiation to be received is very broad, optical systems composed of mirrors are preferred over those composed of refractive lenses. Such reflective optical systems often comprise primary, secondary and tertiary mirrors in which electromagnetic radiation from a source is received and delivered to the primary mirror. The primary mirror then passes the electromagnetic radiation onto the second mirror, which in turn passes the electromagnetic radiation onto the tertiary mirror. The electromagnetic radiation received by the tertiary mirror is then focused onto the infrared detector array which generates electrical signals in response to the radiation received.
Reflective optical systems employing three mirrors are of particular interest in that such systems generally permit correction for the three fundamental types of geometrical aberrations: spherical aberrations, comas, and astigmatism. Spherical aberration is an aberration caused by the spherical form of the mirror that gives different foci for central and marginal rays. Coma is a manifestation of aberration in which the image of a point source not on the axis of the mirror produces a comet-shaped blur. Astigmatism is a defect in an optical system in which rays from a single point of an object fail to meet in a single focal point thus causing the image of a point to be drawn out into a line. When one of the three mirrors in the system is configured so as to provide negative optical power, aberrations due to the curvature of the field may also be corrected.
In certain applications, optical systems must reject and block undesired stray radiation. Failure to reject and block this stray radiation generally results in high levels of noise and spurious signals which inhibit the detection of the desired radiation. Optical systems which provide the most effective means for rejecting stray radiation are termed re-imaging (or relayed) optical systems. Re-imaging optical systems generally have two characteristics: (1) the presence of a real intermediate scene image located within the optical system where a restricting aperture known as a "field stop" can be placed, and (2) the presence of the real image of the system entrance pupil located within the optical system where a restricting aperture known as a "aperture stop" can be placed. Re-imaging three-mirror optical systems are generally preferred for those applications where a high level of aberration correction and a high level of stray radiation rejection are encountered.
While known re-imaging optical systems are generally effective in delivering electromagnetic radiation to a detector array, they often had several disadvantages associated with their use. Chief among these disadvantages are limitations on the useful field of view which can be imaged. The limitations on the use of field view would often result from one or more of the following situations:
(a) ray interferences, vignetting, lack of appropriate clearances, and excessive mirror sizes
(b) excessive geometrical aberrations which prevent the formation of an acceptable image
(c) distortion of the image field which alters the relationship between the object and the image of that object
(d) high angles of incidence of the image F-cones onto the focal plane which can interfere with proper detector and spectral filter operation
(e) excessive collecting area variation resulting from aberrations in the process of re-imaging the entrance pupil to the aperture stop
Several of these limitations are directly related to the magnification at which entrance pupil is re-imaged to the size of the aperture stop. The higher this magnification is, the smaller the aperture stop is relative to the entrance pupil, and the larger of the field of view at the aperture stop is relative to the object space field of view of the system.